An automobile body is equipped with a driving force transmission mechanism having a plurality of rotation shafts and a constant-velocity joint disposed therebetween to transmit a driving force from an engine such as an internal-combustion engine to tires. For example, a drive shaft and a hub are displaceably connected with each other through a Birfield constant-velocity joint, which has an outer race member, an inner race member, and a ball for use in a constant-velocity joint (which may be referred to as a rolling ball) disposed between the races.
Recently, for the purpose of improving the specific fuel consumption of the automobile, weight reduction of a body of the automobile and also various components of the automobile has been studied. In view of the purpose, also the Birfield constant-velocity joint is required to be further lightened (i.e., downsized).
However, small members are generally poor in rigidity. Thus, size reduction of the outer race member, the rolling ball, or the like in the Birfield constant-velocity joint results in lowered durability. In view of solving the problem, in a method proposed in Japanese Laid-Open Patent Publication No. 2000-145804, the surface roughnesses of ball grooves in the outer and inner race members are controlled at 10 to 30 μm, and a lubricant film is formed after a chemical conversion treatment.
In terms of effectively increasing the life of the rolling ball, in a method disclosed in Japanese Laid-Open Patent Publication No. 2002-122145, the rolling ball is composed of a bearing steel or an equivalent material thereof, and the rolling ball is subjected to a nitriding treatment for increasing the surface residual austenite content, and then subjected to a treatment for increasing the crushing load resistance.
In the conventional method described in Japanese Laid-Open Patent Publication No. 2002-122145, when the rolling ball is subjected only to the nitriding treatment, the resultant rolling ball is poor in the crushing load resistance and is easily cracked actually. The crushing load resistance-increasing treatment is carried out for this reason (see, Paragraph 0005). In Japanese Laid-Open Patent Publication No. 2002-122145, a tempering treatment in the temperature range of 180° C. to 230° C. is described as a specific example of the crushing load resistance-increasing treatment. In this example, though the rolling ball is made brittle due to the hardness increase in the nitriding treatment, the surface hardness is lowered to an HRC (Rockwell C-scale hardness) of 60 to 64 in the tempering treatment in the above temperature range.
As a result of intense research, the inventors have found that the surface of the rolling ball is often peeled off in the methods described in Japanese Laid-Open Patent Publication Nos. 2000-145804 and 2002-122145. In other words, it is difficult to maintain sufficient durability of the rolling ball in the conventional methods described in Japanese Laid-Open Patent Publication Nos. 2000-145804 and 2002-122145.
The surface hardness of the rolling ball is lowered in the conventional method described in Japanese Laid-Open Patent Publication No. 2002-122145. However, the rolling ball is required to be abrasion-resistant, whereby it is generally preferred that the rolling ball has a higher surface hardness. As the surface hardness is increased, the abrasion resistance of the rolling ball is improved.